The metal plate bending device of the above-described type is known in, for example, the following Patent Documents 1-3. A metal plate to be bent is placed on the upper plates of a pair of swingable members (or plate supporting members secured thereon, the same shall apply hereinafter) at a starting position wherein the upper plates of the swingable members become flush with each other (which is a position shown in FIG. 1 of Patent Document 1, FIG. 2 of Patent Document 2 and FIG. 3 of Patent Document 3). When an upper die is moved downward to push a center position between the swingable members, the swingable members are rotated in opposite directions so that the metallic plate is bent at the center position. When compared with the device of a time-honored type using a stationary lower die (such as shown in FIG. 3 and FIG. 4 of Patent Document 1), this will provide advantages such as improved machining accuracy.
However, as the metal plate is being bent from its original flat form, its outer surface side will expand due to its material plastic deformation, resulting in out-of-position with respect to the swingable members on which the metal plate is mounted. This will cause formation of scratches on the outer surface of the metal plate and lower its commercial value.
Patent Document 4 proposes, as a solution to this problem, to use a plate supporting member slideably mounted on each of the substantially semi-circular cross-sectional swingable members. More specifically, the plate supporting member 21 is mounted onto the upper surface of the swingable member 15 in such a manner that each upper surface (the metal plate supporting surface) of the swingable member 15 is processed to form a slot 23, and a fastener 25 passing through the slot 23 is screwed to the swingable member 15, thereby allowing the plate supporting member 21 to slide and move in in-and-out directions with respect to the upper surface of the swingable member 15. In such a mounting manner, when the metal plate W supported on the plate supporting members is elongated due to its plastic deformation during the bending operation, the plate supporting member 21 will also move responsively. This will prevent scratches, which would otherwise be formed due to a position shift between the outer surface of the metal plate W and the upper surfaces of the plate supporting members 21.